The dopamine transporter/cocaine receptor (DAT) is the site at which cocaine exerts rewarding/reinforcing effects. Cloning of the rat DAT cDNA by Addiction Research Center molecular neurobiologists during the preceding FY provided an opportunity to study the structure, genetics, and expression of the human DATs and their genes and to seek species differences in structure, function, or interaction with cocaine. Polymorphic markers for the human gene would allow assessments of possible contributions of population variants at the human DAT gene locus to interindividual differences in vulnerability to substance abuse. Studies of the murine DAT gene could facilitate work in transgenic mice. cDNA libraries from human substantia nigra and brain stem, and libraries constructed from human genomic DNA and mouse embryonic stem cell genomic DNA yielded human and murine cDNAs and genomic clones; the human DAT is 94% identical to the rat sequence. It contains 620 amino acids that can be arrayed in 12 hydrophobic, putative transmembrane domains with three potential sites for N-linked glycosylation. The human DAT gene lies on the telomeric region of chromosome 5pl5.3. Restriction Fragment Length Polymorphisms (RFLPs) and Variable Number Tandem Repeat (VNTR) genetic markers for the human DAT gene were identified along with racially-dimorphic population distributions of the RFLP marker. Studies of both markers in the same individuals reveal no detectable linkage disequibrilium. DNA from individuals whose drug use histories were characterized at the ARC was examined to seek association between substance use and these 3' and 5' DAT gene markers. The VNTR and Taq I RFLP were examined in more than 170 unrelated individuals with well-characterized histories of lifetime substance use. These same individuals show substantial allelic association with markers at the 3' and 5' ends of the dopamine D2 receptor gene. However, there was no association between either DAT gene marker and substance abuse. Human and murine DAT genomic clones reveal intron/exon boundaries at several sites conserved with respect to the GABA transporter gene.